How would weather on a Cube-World with uniform surface gravity behave?

Question

In a cube-world with uniform surface gravity with mildly rounded corners, how would weather behave? The Cube-world is roughly earth-sized, with the edges rounded by about a hundred kilometers. Gravity is roughly earth-normal across the entire surface.

I'm mostly wondering about whether weather systems would span between faces (having to turn 90 degrees, despite gravitational assistance), whether there would be intense perpetual or frequent storms near the edges, and whether the geography of the edges (Ocean/land, mountain/flat) would cause different effects than on a spherical world.

The planetary tilt relative to the sun would be roughly that of earth (or perhaps a bit less), with a corner of the cube as the "North" and "South" poles.

Answer 1

This is a highly improbable scenario, which requires a lot of hidden assumptions to work, but lets roll with it.

First of all, while the planet is a cube, there is no way to have cubic atmosphere (atmocube?), unless you magic the gasses to act differently. If you have a cubic planet within a spherical atmosphere, then either the edges and corners extend beyond the atmosphere, or not. If they extend into the vacuum of space, then there would be 6 distinct atmospheres, all very shallow and sloshing about, pretty inimical to life and cool stories to be set there.

If the atmosphere is big enough to encompass the whole cube, fun shenanigans occur. The atmoshpere will be much thicker at the centers of each face of the cube, and much thinner near the edges. This would cause a big difference in pressure. Since all faces would have the same issue, the edges will experience hurricane winds from the planar air masses clashing. Worse still on the "equatorial" corners, which would experience hurricane winds from 4 directions at once. An advanced civilization could easily use the corner to launch stuff into space, on wind-power alone.

The same "edgewise" winds will pull moisture with them, which would then shear off over the edge, and come down as tremendous rainfall.

Long story short, the edges and corners would be absolute tempestuous hell on wheels.

The "polar" corners will be similar to our Antarctica, and rounded with a cap of ice.

The centerpoint of every face of the cube will be sandblasted plains/deserts with little water, and unpleasantly high air pressure. The mid-point between the center and the edge will be covered in a roughly square grid of small seas, lakes and rivers, and this are will be the most livable.

The day-night cycle would be quite dramatic, with the sun suddenly bursting from behind the edge, and vanishing just the same. Days would be much hotter, nights much colder, and the dawn and the dusk would be punctuated by even more dramatic weather (not that anyone would care, as the Cube planet would experience the kinds of weather we would consider "apocalyptic" on a spherical planet anyway).

Notably, a cubical planet would want to become a sphere due to entropy. Even if we discount any kind of sensible plate tectonics, the edges will slowly erode due to the wind and rain blasting them at hypercane speeds. This means that at places where the face-winds do not match exactly, one side will be blasted by a rain of mud, sand and small rocks carved away from the opposite face with the waterjet. So, imagine a storm that not only takes the entire horizon, combines a hypercane with a flying tsunami wave, vertical rainfall, rivers being pushed uphill by wind, but also rains mud and sand, and occasionally whips sand around with enough force to sandblast you to death.

All in all, a pretty cool planet for skydiving sports!

Answer 2

Note: this answer presumed a cube with roughly homogenous content, not one where the gravity had been flattened out. To me, the phrase “Gravity is roughly earth-normal across the entire surface” just meant that it was roughly Earth-sized, not exotic crust design. I apologize if I misunderstood the question. Flat gravity would be very different answer!

It is a mistake to think of cube world as having six flat faces. Gravity pulls toward the center always. Instead, cube world models as a sphere with eight mountain ranges, one on each edge. Prevailing winds do not blow across a flat plain — they blow up a mountain! The right mental model is to think of the center of each cube face as a depression in a bowl with edges. You can more easily visualize this by considering the distance of each surface point to the center of the sphere. Even though the land is a cube, the atmosphere will be a sphere! If the atmosphere is small enough, the “peaks” of the cube edges will be out in space. Yours are rounded by wind, so your atmosphere is larger than the cube.

There won’t be oceans, just six seas in the face bowls. Water doesn’t flow uphill! That’s what you need to focus on: anyone walking toward an edge will feel like their walking uphill; towards a center will feel like walking downhill.

The “weather” will be largely gathered in those bowl spots because that’s where you have churn. Sometimes it will spill over a mountain, which will cause a giant chinook wind as one face spills onto another, just like what happens with the jet stream and Rocky Mountains. Each of your faces will develop based upon how much sunlight that face receives.

With an Earth axis tilt of 23 degrees and axis running through two faces, you’ll have two polar faces and four equatorial faces. If you run the pole between two corners, you’ll have six non-polar faces. In both models, you’ll have seasonal progression from more/less sunlight. In both models, the non-polar faces are probably similar unless there’s other geography making them different: a large volcano. Or maybe one never accumulated a sea, for some reason, so its dynamics are less stable.

Hope that helps.

PS: viewed from space, a sea on a cube face would look like a raised bubble! The water will try to pull into a sphere! You won’t be able to look across a face because of the raised dome of sea in the middle. Sailing on such a sea will feel flat, just as it does on Earth’s oceans.

PPS: Earth’s Moon causes lots of our weather. I didn’t include a moon in my write up above. If you add a moon, that moon will raise and lower those sea domes as it orbits. It’ll also pull on the atmosphere, and it can cause the chinooks by pulling atmosphere higher than the mountain edges. A moon that orbits very close to the rotation speed of the planet would cause interesting seasonal effects as a given face would hang out under the moon for a while.